Treatments for patients having substantially impaired renal function, or kidney failure, are known as "dialysis." Either blood dialysis ("hemodialysis") or peritoneal dialysis methods may be employed. Both methods essentially involve the removal of toxins from body fluids by diffusion of the toxins from the body fluids into a toxin free dialysis solution.
Hemodialysis involves removing blood from the patient, circulating the blood through a dialysis machine outside the body, and returning the blood to the patient. As the blood is directly in contact with the hemodialysis membrane, the patient ordinarily needs to be treated only 3-5 hours at a time, about three times per week. Unfortunately, hemodialysis requires the use of complex and expensive equipment, and can therefore normally only be performed under controlled conditions of a hospital or other specialized clinic.
Peritoneal dialysis, on the other hand, can be performed where such complex equipment is not readily available, such as in the patient's home. In the peritoneal dialysis process, the patient's peritoneal cavity is filled with a dialysate solution. Dialysates are formulated with a high concentration of the dextrose, as compared to body fluids, resulting in an osmotic gradient within the peritoneal cavity. The effect of this gradient is to cause body fluids, including impurities, to pass through the peritoneal membrane and mix with the dialysate. By flushing the dialysate from the cavity, the impurities can be removed.
Due to indirect contact with bodily fluids through bodily tissues, rather than direct contact with blood, the dextrose concentration needs to be considerably higher in peritoneal dialysis than in hemodialysis, and the treatment is generally more prolonged. Peritoneal dialysis may be performed intermittently or continuously. In an intermittent peritoneal dialysis (IPD) procedure, the patient commonly receives two liters of dialysate at a time. For example, in a continuous ambulatory peritoneal dialysis (CAPD) procedure, the peritoneal cavity is filled with two liters of dialysate and the patient is the free to move about while diffusion carries toxins into the peritoneal cavity. After about 4-6 hours, the peritoneum is drained of toxified dialysate over the course of an hour. This process is repeated two to three times per day each day of the week. Continuous Cycle Peritoneal Dialysis (CCPD) in contrast, involves continuously feeding and flushing dialysate solution through the peritoneal cavity, typically as the patient sleeps.
Because peritoneal dialysates are administered directly into the patient's body, it is important that the dialysis solution maintains the correct proportions and concentrations of reagents. Moreover, it is impractical to formulate and mix dialysis solutions on site at the typical location of administration, such as the patient's home. Accordingly, peritoneal dialysates are typically delivered to the site of administration in pre-mixed solutions.
Unfortunately, dialysis solutions are not stable in solutions over time. For example, dextrose has a tendency to caramelize in solution over time, particularly in the concentrations required in the peritoneal dialysis context. To prevent such caramelization, peritoneal dialysis solutions are typically acidified, such as with hydrochloric acid, lactate or acetate, to a pH between 4.0 and 6.5. The ideal pH level for a peritoneal dialysate, however, is between 7.2 and 7.4. While achieving the desired goal of stabilizing dextrose in solution, the pH of acidified peritoneal dialysis solutions tends to damage the body's natural membranes after extended periods of dialysis. Additionally, the use of acidified peritoneal dialysates tends to induce acidosis in the patient.
Bicarbonates introduce further instability to dialysis solutions. The most physiologically compatible buffer for a peritoneal dialysate is bicarbonate. Bicarbonate ions react undesirably with other reagents commonly included in dialysate solutions, such as calcium or magnesium in solution, precipitating out of solution as insoluble calcium carbonate or magnesium carbonate. These insolubles can form even when the reactants are in dry form. When occurring in solution, the reactions also alter the pH balance of the solution through the liberation of carbon dioxide (CO.sub.2). Even in the absence of calcium or magnesium salts, dissolved sodium bicarbonate can spontaneously decompose into sodium carbonate and CO.sub.2, undesirably lowering the solution's pH level.
The current alternatives to bicarbonate for buffering peritoneal dialysate are acetate and lactate, but these reagents also have undesirable chemical consequences. For example, there is some evidence that acetate may reduce osmotic ultrafiltration and may induce fibrosis of the peritoneal membrane.
The incompatibility of reagents commonly found in dialysates thus creates significant logistical problems in connection with their preparation, storage and transportation. Attempted solutions to these problems have included various devices and methods for providing dry formulations of reagents, and for separately storing and dissolving incompatible reagents. See, e.g., U.S. Pat. Nos. 4,467,588, 4,548,606, 4,756,838, 4,784,495, 5,344,231 and 5,511,875. Many of these proposed systems involve elaborate water pumping and re-circulation systems, pH and conductivity monitors and water heating components. Moreover, sterile water must be provided independently, further complicating the formulation process.
While many prior methods and devices have been successful to one degree or another in addressing logistical problems, they have proven unsatisfactory for various reasons. Conventional systems are quite complex and expensive, such that they are impractical for many settings. Thus, dialysate solutions still tend to be prepared well in advance of administration, risking destabilization and/or requiring acidification of the solutions, as noted above. Additionally, pre-formulated solutions are quite bulky and involve considerable transportation and storage expense.
Accordingly, a need exists for improved methods and devices for formulating solutions for peritoneal dialysis. Desirably, such methods and devices should avoid the problems of non-physiologic solutions and incompatibility of dialysate reagents, and also simplify transportation, storage and mixing of such dialysates.